1. Field of the Invention
The present invention relates to a water base ink for ink-jet recording.
2. Description of the Related Art
The ink-jet recording method is a printing technique in which ink droplets are formed by the ink discharge method including, for example, the electrostatic attraction method, the method in which mechanical vibration or displacement is applied to the ink by using a piezoelectric element or the like, and the method in which bubbles are generated by heating the ink to utilize the pressure generated thereby. All or a part of the ink droplets are landed on a recording objective material such as paper to perform the recording.
In the ink-jet recording method, dye inks using a dye as a coloring agent have been hitherto widely used for the following reason. The dye ink hardly causes inconveniences such as deposition of precipitates because the dye is dissolved in the ink, and the ink is relatively excellent in handling performance.
In the recent years, however, pigment inks using a pigment such as carbon black have been used as the coloring agent for the following reason. The pigment is more effective than the dye at enhancing the density of the image area although the pigment has a lipophilic surface and thus is not excellent in dispersion stability in water. Accordingly, with the pigment, it is possible to enhance the contrast between the image area and the non-image area for the following reason. The dye is dissolved in the ink at the molecular level and thus behaves in the same manner as a solvent used in the ink and permeates into a recording objective material such as paper. On the other hand, pigment particles constituting the pigment are prohibited from moving (permeating) by, for example, the additives and fibers contained in the recording objective material. Therefore, the pigment particles tend to remain on the paper surface, thereby enhancing the concentration of image and line areas and the contrast of these areas. In addition, the pigment ink is more effective than the dye for the problem of “blurring”. The blurring refers to such a phenomena that the ink nonuniformly spreads along the surface of recording objective material when the ink permeates thereinto. As a result, the edge of an image area and/or line area become jaggy. The pigment ink is more effective than the dye ink in avoiding the blurring because the pigment ink is dispersed in the ink as the solid content whereas the dye ink is dissolved in the ink at molecular level. Thus, with the pigment ink, the sharp edge of image area and/or line area can be obtained more easily than with the dye ink.
The pigment ink, however, is inferior to the dye ink in dispersion stability in water as described above. Accordingly, there are following problems associated with the pigment ink. When an ink-jet recording apparatus with the pigment ink is left to stand for a long period of time after performing an ink-jet recording, there are problems in some cases such that the pigment particles in the pigment ink are aggregated in the interval between ink-jet recording operations and/or even during the next ink-jet recording operation; the aggregated particles cause a clog-up of a nozzle of the recording head of the apparatus; the aggregated particles adhere or firmly fix to the surrounding of nozzle, thereby damaging a repellent ink coat, which in turn causes the discharge failure such as non-discharge and incorrect discharge in ink flying direction (bending). In view of these problems, there is a demand to prevent the discharge failure and to improve the discharge stability regarding the amount of ink droplet, the discharge speed or the like. For this purpose, various methods have been devised.
For example, there is a method in which the surface of carbon black is chemically treated so as to introduce a functional group such as carboxyl group and sulfonic group, thereby providing the water dispersiblity to the carbon black. The chemically treated carbon black has a self-dispersiblity in which the carbon black (pigment) exhibits the negative zeta potential. The ink using this self-dispersible carbon black, however, has the following problem. When the ink contains cations such as sodium ion, potassium ion, calcium ion and magnesium ion, the cations are bonded to the negative charge on the surface of carbon black particles, and the electric charge becomes zero. As a result, the electric repulsive force among the particles of carbon black is lost and the aggregation of the particles is easily caused.
In order to solve the problem, in the ink using the self-dispersible black ink, it has been devised to control the quantity of impurities of the cations in the ink so as to prevent the aggregation of carbon black.
U.S. Pat. No. 5,637,140 corresponding to Japanese Patent Application Laid-open No. 9-3374 discloses to control the total content of fatty acids, such as palmitic acid and stearic acid, contained as impurities in the ink to be not more than 0.3% by weight so as to prevent the clogging of nozzles, the incorrect discharge in ink flying direction, the change in ink drop quantity and the changes in flying speed of the ink which would be otherwise caused by the insoluble salts of the fatty acids. Japanese Patent Application Laid-open Nos. 2003-138179 and 2003-147235 disclose to control the total content of a fatty acid and a fatty acid salt to be not more than 0.5% by mass to lower the coefficient of redispersion of the ink and thus improve the dispersibility, thereby preventing the pigment particles from excessively moving on the surface of recording objective material and improving the abrasion resistance or the like. In Japanese Patent Application Laid-open Nos. 2003-138179 and 2003-147235, the fatty acid is defined as a saturated or unsaturated alkylcarboxylic acid having a number of carbons of 8 to 22 such as lauryl acid and paltimic acid, and that the fatty acid salt is defined as a salt formed of the above-defined fatty acid and an organic or inorganic cation. In addition, Japanese Patent Application Laid-open No. 2003-138179 discloses that a pigment particle has a polar group such as carboxyl group and sulfonic group on the surface thereof.
However, in the water ink for ink-jet recording containing the self-dispersible carbon black as the coloring agent, the discharge stability could not be sufficiently improved even by lowering the total content of cations, the higher fatty acid such as palmitic acid and stearic acid and the salt thereof, respectively contained in the ink as impurities. In addition, the ink jet printer of the recent model has an increased number of nozzles which are minuter than conventional nozzles in order to obtain more highly defined images. Under the circumstances, it is demanded more than ever to further improve the discharge stability from the nozzles.